This tutorial was created for Photoshop CS2.
Any other programs may or may not work. :D
1. Open up an image.
2. Click the paintbrush tool.
3. Select the 200 or 300 pixel brush, with 0% hardness. Set the mode to screen.
4. Click anywhere on the picture. :D Just don't use the color black..haha. It won't show up. Everything else will!
5. You're done. :)